1. Field of the Invention
The present invention relates to a developing toner for electrophotography which visualizes an electrostatic charge latent image formed in electrophotography, electrostatic printing, electrostatic recording, etc. and to an electrophotographic device. In particular, the invention relates to an electrophotographic device for transferring a toner image on a photoreceptor onto a recording medium by a transfer unit and then cleaning the residual toner on the photosensitive element by a cleaning blade.
2. Description of the Related Art
Among the foregoing printing or recording methods, for example, in the electrophotographic method, a photoconductive photoreceptor is charged and exposed to form an electrostatic charge latent image on the photoreceptor, the electrostatic charge latent image is then developed with a finely divided toner containing a resin as a binder and a coloring agent, etc., and the resulting toner image is transferred onto recording paper and fixed thereonto to obtain a recording image.
In such electrostatic image recording steps, in order to always obtain a beautiful image quality, it is important that the residual toner in the preceding step is not carried over into the next step, and therefore, a cleaning step takes an especially important step.
So far, as a method of developing a toner, a magnetic brush development method using a two-component developer composed of a toner capable of achieving high-speed and high-image quality development and a magnetic carrier has been frequently employed. Also, as a method of cleaning the residual toner, a method of contacting and rotating a brush to undergo mechanical cleaning and a method of press contacting a blade and scraping off the residual toner to achieve cleaning have hitherto been put into practical use. Above all, the latter cleaning mode by a blade is most generally employed because it does not require a drive mechanism, etc. and is cheap.
On the other hand, in recent years, a laser beam printer using laser beams for exposing a photoconductive photo-sensitive element and reproducing a recording image with dots by a modulation signal due to indication of a computer is developing with the development of information instruments. In particular, in the latest laser beam printers, since the image preparation with a higher image quality is demanded, the diameter of laser beam is contracted and made small so that the dot density becomes high as 600 to 1,200 dpi (dots/inch). Following this matter, for the purpose of developing an electrostatic charge latent image which has become fine, the particle sizes of toner and carrier become small, and it is advanced to apply a small-particle size toner having a volume average particle size of not more than 10 μm and a small-particle size carrier having a weight average particle size of not more than 100 μm.
However, in the small-particle size toner, since the yield is lowered in the pulverization and classification steps at the time of toner production, costs of the toner become high. In the small-particle size toner, such a problem is caused, and in general, it is difficult to put a toner of less than 4 μm into practical use. Accordingly, a toner prepared by making the average particle size fall within the range of from 4 to 10 μm, thereby cutting off fine particles and enhancing the fluidity by improving external additives and external preparation of the toner is used.
On the other hand, with the reduction of the particle size of the toner, the carrier is made small so as to have a weight average particle size of not more than 100 μm, and the specific surface area of the carrier is increased, whereby frictional electrification properties are enhanced. However, in a carrier of less than 30 μm, a magnetic force of the carrier is lowered, and the carrier is liable to adhere onto an electrostatic charge image holding member due to an electrostatic attraction. Accordingly, a carrier prepared by classifying it into an average particle size in the range of from 30 to 100 μm and optionally coating the surface with a resin is used.
By an improvement of the particle size distribution and improvements of the fluidity and electrification, small-particle size toners and developers have become put into practical use in copying machines, printers, and so on. However, in the case of carrying out printing in an actual machine, especially in the case of repeating printing of 10 sheets or more per minute, the foregoing problems inherent to the small-particle size toners are caused, and a lowering of the life of a developer due to carrier spent by the toner and a lowering of the life of a photoreceptor due to photoreceptor filming by the toner are likely generated.
Also, the small-particle size toners have strong electrostatic adsorptivity and are liable to generate cleaning failure. Accordingly, so far, a device such as an increase of contact pressure of a blade was made. However, because of an increase of the coefficient of friction against the photoreceptor, the generation of a scratch, a lowering of the life, and the generation of stripping of the blade possibly occurred. In the case where a scratch is generated, a scoreline is generated on the image, resulting in printing failure. Also, in the case where stripping of the blade is generated, the residual toner slips through the blade and is carried over into the next step, resulting in the occurrence of printing staining. In particular, in the case where the electrostatic adsorptivity is especially strong, the toner is hardly scraped off by the blade, and therefore, the blade is more strongly pressed against the photoreceptor, thereby trying to scrape off the residual toner. Thus, the blade is more likely stripped.
In order to prevent such problems from occurring, a method of controlling the adsorptivity of the toner and a method of controlling the toner amount to be fed as a lubricant between the blade and the photoreceptor to prevent stripping of the blade from occurring have been proposed. However, for the purpose of enhancing the scraping effect, it was difficult to cope with the life. Therefore, it was necessary for bringing more softly the blade with press contact with the photoreceptor to achieve a long life, thereby preventing the occurrence of printing staining due to scraping failure of the residual toner.
Also, so far, with respect to the blade cleaning, no improvement has been made from the standpoint of toner material; no improvement has been made while turning an attention to a wax as a mold releasing agent for optimizing the adsorptivity of the toner; and an improvement has been made while mainly taking the main purpose for the thermal characteristic.
JP-A-10-49021, JP-A-11-45035, and JP-A-2000-242041 can be enumerated as such known documents.